Add "--check-heads" option to "current" command

Add "--check-heads" option to "current" command

Added :paramref:`.command.current.check_heads` parameter to
:func:`.command.current` command, available from the command line via the
``--check-heads`` option to ``alembic current``.  This tests if all head
revisions are applied to the database and raises :class:`.DatabaseNotAtHead`
(or from the command line, exits with a non-zero exit code) if this is not
the case.  The parameter operates equvialently to the cookbook recipe
:ref:`cookbook_check_heads`. Pull request courtesy Stefan Scherfke.

Fixes: #1705
Closes: #1739
Pull-request: https://github.com/sqlalchemy/alembic/pull/1739
Pull-request-sha: 955f9b7ff6216c3c2940597a652cce538a6e0f29

Change-Id: I1173217ff6a4927226ec9e7b8c7ef67c6431d728